Chemical composition of manganese nodules from the Pacific Ocean

Manganese nodules from six Pacific Ocean sites contain two chief oxide components which can be distinguished by both optical and X-ray diffraction methods. Optically coordinated electron microprobe spot analyses reveal that copper and nickel are concentrated in the manganese-rich crystalline oxides (birnessite and todorokite) while cobalt is concentrated in the iron-rich amorphous oxides. In the nodules studied the maximum nickel content in the crystalline material is 3.6%, while the maximum copper content is 2.95%. The highest cobalt content found in the amorphous material was 0.74%. Large variations in concentrations were found within individual nodule layers as well as from one layer to another. Most of the variations cannot be accounted for by admixed nodule components. The variations cannot be distinguished by any optical or X-ray diffraction criteria and are probably controlled by the availability of the elements during primary deposition. There does not appear to be any linear interelement relationship between concentrations of the major elements (Mn and Fe) and the minor elements (Ni, Cu and Co). Linear relationships that have been reported in the past must be interpreted with care, for crystalline oxides and amorphous oxides may have been mixed in varying amounts during the sampling. In view of the different element associations in the crystalline and the amorphous oxides, the main factor for controlling bulk chemical analysis of a particular nodule appears to be the ratio of crystalline to amorphous material in the nodule. NSF-IDOE support of this research is acknowledged with thanks.